Servo system that is insensitive to tape flutter errors



Sept. 6, 1966 D, J. COCHRAN ETAL 3,271,526

SERVO SYSTEM THAT IS INSENSITIVE TO TAPE FLUTTER ERRORS Filed Nov. 5, 1962 United States Patent 3,271,526 SERVO SYSTEM THAT IS INSENSITIVE TO TAPE FLUTTER ERRORS Donald James Cochran, Pacifica, and Donald Marvin Patterson, Sunnyvale, Calif., assignors to Ampex Corporation, Redwood City, Calif., a corporation of Caliorma Filed Nov. 5, 1962, Ser. No. 235,376 6 Claims. (Cl. 179-1002) This invention relates t-o speed control servo systems, and more particularly to improved speed control servo systems for magnetic tape record-playback apparatus.

In magnetic tape recording and playback of high frequency signals, such as television signals, a speed control servo system is needed to ensure that longitudinal tape position relative to a head drum position is exactly the same in a playback mode as it was in a record mode. The speed control servo system locks the tape motion from reel-to-reel to the rotation of the head drum so that a previously recorded control track will pass over a control track head. During the record mode, a pulse generating means generates a pulse for each revolution of the head drum. The pulses are amplified and then recorded along one margin of the magnetic tape by the control track head. During playback, the control track pulses on the magnetic tape are reproduced and fed into a ramp generator for generating a ramp pulse for each control track pulse. While the control pulses on the magnetic tape are being reproduced, the pulse generating means is generating a sampling pulse for each revolution of the head drum. Such head drum sampling pulses generated during playback are fed into a differentiating network to produce sharp sampling pulses. Each sharp sampling pulse is compared with one ramp pulse and any dil'ference in time between the two pulses is detected by an error detector to produce an error voltage which is used to speed u-p or slow down the head drum motor. Therefore, longitudinal tape position relative to the head drum position is exactly the same during the playback mode as it was during the record mode.

In the past, a speed control servo system utilizing a sampled ramp time comparator has been used to synchronize the position of a head vdrum relative to the position of a magnetic tape during playback. The loop gain of the speed control servo system is proportional to the slope of a ramp pulse waveform. Thus, a ramp pulse waveform having a steep slope provides a high gain servo system which locks the tape motion from lreel-to-reel to the rotation `of a :head drum. The use of a high gain servo system, however, although desirable to improved synchronization, give rise to certain disadvantages and drawbacks, such as making the locked-in servo system very sensitive to tape utter errors. The high loop gain of the locked-in servo system amplies the tape flutter errors which cause the speed of a head drum motor to change randomly. Tape ilutter errors, therefore, pull the lockedin servo system out of synchronization, thereby distorting a video picture. Thus, prior high gain se-rvo systems were very sensitive to tape flutter errors which caused locked-in servo systems to be -pulled out of synchronization.

It is, therefore, the principal aim yof this invention to provide an improved magnetic tape servo system that does not have the disadvantages and drawbacks of prior magnetic tape servo systems. This is accomplished by providing a diode pulse shaping means for shaping ramp pulse waveforms so that they have flat sections at their zero crossover points. The output voltage of the diode pulse shaping means is zero in the fiat sections due to the saturation voltage drops appearing across the diodes. The width of the flat section of a ramp waveform curve is determined by the number of diodes used in the pulse shap- 3,271,526 Patented Sept. 6, 1966 ing means. Since the loop gain of the servo system in the at sections is zero, the servo system does not detect tape flutter errors falling within these sections. The servo system, therefore, remains locked-in for the entire fiat section of each ramp pulse curve. Thus, the present invention provides a simple and inexpensive way of making a magnetic tape servo system insensitive to tape flutter errors so that they do not pull the locked-in servo system out of synchronization.

It is therefore, the main object of this invention to provide an improved speed control servo system for a magnetic tape record-playback apparatus.

It is another object of this invention to provide a simple and inexpensive way of making a speed control servo system insensitive to tape flutter errors.

Other objects and advantages of this invention will be apparent from the following detailed description of a preferred embodiment of the invention when taken with the drawing which shows in the sole figure a schematic diagram of the preferred embodiment of the invention.

Referring now to the sole ligure, there is shown a speed control system for a magnetic tape record-playback apparatus. During the playback mode, previously recorded pulses on a magnetic tape (not shown) are reproduced by a control track head (not shown), The waveform of the control track output is illustrated by reference numeral 10. The reproduced control track pulses are applied to an input terminal 12 and then fed into a ramp generator 14 for generating a ramp pulse for each control track pulse. The output waveform of the ramp generator 14 is illustrated by reference numeral 15 (two being shown). The sloping portion of the ramp pulses of each waveform 15 have zero crossover points 16 and 17, respectively.

In accordance with the present invention, there is provided a -pulse shaping network 20 comprising four diodes 21, 22, 23 and 24. The pulse shaping network 20 shapes the ramp pulse curves so that they have flat sections 26 and 27 at their zero crossover points 16 and 17, respectively. The output waveform of the pulse shaping network 20 is illustrated by reference numeral 2S. The output voltage of the pulse shaping network 20 is zero in the flat sections 26 and 27 due to the saturation voltage drops appearing across the diodes 21, 22, 23 and 24. The width of each of the flat sections 26 and 27 of the ramp pulse curves is determined by the number of diodes used in the pulse shaping network 20. The loop gain of the speed control servo system is proportional to the slope of the ramp pulses. Thus, since the slope of the iiat sections 26 and 27 of the ramp pulses is zero, the loop gain of the servo system for these sections 26 and 27 is also zero. The servo system, therefore, does not detect tape flutter errors and remains locked-in for the flat sections 26 and 27 of the ramp pulse curves.

While the control track pulses, as illustrated by waveform 10, are being reproduced, a pulse generating means (not shown) is generating a sampling pulse yfor each revolution of a head drum (not shown). The waveform of the head `drum output is illustrated by reference numeral 11. The head drum sampling pulses are applied to input terminal 13 and then fed into a differentiating network l30` to produce sharp sampling pulses. The output Waveform of the differentiating network 30 is illustrated lby reference numeral 31. Each sharp sampling pulse is compared fwith one yramp pulse and any difference intime betrween the two .pulses is detected 'by an error detector 32 4to produce a positive or negative error voltage which is used to speed up or slow down a head drum motor (not shown). However, tape diutter errors due to the tape sloiwing 4down or speeding up, as represented by dotted sampling pulses, fall within the flat section 26 and 27. Therefore, time diierences between the dotted sampling pulses and the ramp pulses are not detected by the error detector 32. Thus, the speed -control servo4 system is insensitive t0 `tape flutter errors and remains locked-in for the flat sections 26 and 27 of the ramp pulse curves.

In the record operation, vertical synchronization pulses are stripped from video information by a sync stripping network (not shown). The vertical synchronization pulses are then ted into the ramp generator A14 tov produce ramp pulses. However, since there are no tape ilutter errors during the record mode, the diode pulse shaping network 20 is disabled. A pulse -generating means (not shown) generates a sampling pulse for each revolution of a head drum (not shown). The sampling pulses are amplied and then recorded .along one margin of a magnetic tape (not shown) by a control track head (not shown). At the same time, each sampling pulse is compared with one ramp pulse and any `difference in time between the Atwo pulses is detected by the error detector 32 to produce an error voltage which is used to speed up or slow down a head drum motor (mot shown). Thus, duri-ng the record morde, the 'servo system synchronizes the head drum to each television picture lield.

In the playback operation, control track pulses, as illustrated lby waveform 10, are applied to input terminal l12 and head drum pulses, as illustrated by waveform 11, are applied to input terminal 1'3. The control track pulses are fed into the rra-mp generator 14 to ,produce ramp pulses, as illustrated by waveform 15. The ramp pulses are then fed into the diode pulse shaping network 20 so that the ramp pulses have at sections 26 and 27 at their zero crossover points 16 and 17, respectively. The output voltage ot the diode pulse shaping network 20 is zero in the flat sections 26 and 27 due to the saturation voltage drops appearing across diodes 21, 22, 23 and 24. By way of example, the diodes 21, 22, 23 and 24 are silicon diodes, and each has `a saturation voltage drop of .6 volt. Thus, the width of each ilat section 26, 27 is determined by the total voltage drop of 2.4 volts across the four diodes 21, 22, 23 and 24. Since the slope of the flat sections 26 and 27 of the ramp pulse curves is Zero, the loop gain of the speed control servo system is also zero. Tape flutter errors due to the magnetic tape slowing down or speeding up are represented by time shifting the 'head drum sampling pulses of waveform 11. This is illustrated on the waveform 11 by dotted head drum sampling pulses. These dotted sampling pulses are differentiated by the differentiating network 30 to produce dotted sharp sampling pulses, which are shown on waveform 31. Tape flutter errors due to tape slowing down or speeding up, as represented by dotted sharp sampling pulses, tall within the at sections 26 and 27 of the ramp pulse curves of waveform 25. Since the loop gain of the servo system lfor flat sections 26 and 27 is zero, time differences between the dotted sharp sampling pulses and the ramp pulses are not detected by the error detector 32. The speed control servo system, therefore, is insensitive to tape ilutter errors and remains locked-in for the iiat sec- `tions 26 and 27 of the ramp pulse curves.

Thus, it is seen that the present invention .provides a vsimple and inexpensive `way of making a magnetic tape servo system insensitive to tape flutter errors so that they do not pull the locked-in servo system out of synchronization.

Although the present invention has been shown and described in terms ot a preferred embodiment, changes and modifications which do not depart lfrom the inventive concepts taught herein will suggest themselves to those skilled in the art. Such changes and modifications are deemed Y-to tall within the scope ofthe invention.

What is claimed is: Y 1. A speed control servo system Afor a magnetic tape Yrecord-playback apparatus comprising:

means for differentiating a second group of pulses to produce sharp pulses;

means for comparing the time between one sharp pulse and one ramp pulse; and

means ttor shaping the ramp pulses at their zero crossover points to make the servo system insensitive to tape utter errors.

2. A speed control servo system for a magnetic tape record-playback apparatus comprising:

means for generating ramp pulses tfrom a iirst group of pulses, said ramp pulses having zero crossover points;

means for differentiating a second group of pulses to produce sharp pulses; Y means for comparing the time between one sharp pulse and one ramp pulse; and diode -means for shaping the ramp pulses at their zero crossover points to make the servo system insensitive to tape flutter errors. 3. A speed control servo system for a magnetic tape record-playback apparatus comprising:

ramp generating means for generating ramp pulses from a group olf previously recorded pulses, said ramp pulses having Zero crossover points; differentiating means Ifor diiierentiating a group of sampling pulses to produce sharp pulses; time comparator means for comparing the time between one sharp sample pulse and one ramp pulse; and a diode pulse shaping vnetwork lfor providing the ramp pulses with at sections at their zero crossover points for making the servo system insensitive to tape ilutter errors falling Within the flat sections of the ramp pulses. l 4. A speed control servo system for a magnetic tape record-playback `apparatus comprising:

means for reproducing previously recorded control pulses; ramp generating means for -generating ramp pulses from the previously recorded control pulses, said ramp pulses having zero crossover points; a head drum; pulse generating means for generating a sampling pulse `for each revolution of said head drum; differentiating means for differentiating the head drum sampling pulses to produce sharp sampling pulses; time comparator means for comparing the time between one sharp sample pulse and one ramp pulse; and a diode pulse shaping network -for providing the ramp pulses with iiat sections at their zero crossover points for making the servo system insensitive to tape flutter errors falling Within the flat sections of the ramp pulses. S. A speed control servo system for a magnetic tape record-playback apparatus comprising:

means for reproducing previously recorded control pulses; ramp generating means 'for generating ramp pulses from the previously recorded control pulses, said ramp pulses having zero crossover points; a head drum; pulse generating means for generating a sampling pulse for each revolution of said head drum; d-iierentiating means for differentiating the head drum sampling pulses to produce sharp sampling pulses; time comparator means folcomparing the time between one sharp sample pulse and one ramp pulse; a diode pulse shaping network comprising at least two parallel-connected diodes with diodes ot one branch being poled oppositely to the diodes of the other branch; and said diode pulse shaping network for providing the 5 6 ramp pulses with hat sections at their zero crossover References Cited by the Examiner points for making the servo system insensitive .to UNITED STATES PATENTS phegrtrr egg; fa'umg Wlthm the Hat ctms 3,168,618 2/1965 soudermeyer 31e-314 X pp 3,174,090 3/1965 Hall 318-314 6. A speed control servo system according to cla-im 1, 5 further dened by said means for shaping the ramp pulses BERNARD KONICK, Primary Emmi-nercomprising two parallel-connected `diode branches conl nected to the output of the -lamp pulse Igenerating means, IRVING SRAGOW Exammer each :branch including at least one diode and the diodes of R. M. I-ENNINGS, A. I. NEUSTADT, 'che respective rbnanohes .being oppositely poled. 10 Assistant Examiners. 

2. A SPEED CONTROL SERVO SYSTEM FOR A MAGNETIC TAPE RECORD-PLAYBACK APPARATUS COMPRISING: MEANS FOR GENERATING RAMP PULSES FROM A FIRST GROUP OF PULSES, SAID RAMP PULSES HAVING ZERO CROSSOVER POINTS; MEANS FOR DIFFERENTIATING A SECOND GROUP OF PULSES TO PRODUCE SHARP PULSES; MEANS FOR COMPARING THE TIME BETWEEN ONE SHARP PULSE AND ONE RAMP PULSE; AND DIODE MEANS FOR SHAPING THE RAMP PULSES AT THEIR ZERO CROSSOVER POINTS TO MAKE THE SERVO SYSTEM INSENSITIVE TO TAPE FLUTTER ERRORS. 